Polymeric polypropylene compositions have gained wide commercial acceptance and usage in numerous applications because of the relatively low cost of the polymers and the desirable properties they exhibit. Polypropylene homopolymers, however, have the disadvantage of being brittle with low impact resistance, particularly at low temperatures. Most procedures proposed for modifying the properties of polypropylene homopolymer to improve the impact strength have included the provision of a polypropylene/other .alpha.-olefin copolymer phase in an otherwise homopolymeric polypropylene. A propylene/ethylene copolymer phase is particularly useful for this purpose. The structure of such products is not entirely certain with some sources referring to a block copolymer and other sources referring to other structures. However, such materials are well known and of substantial commercial importance. They are referred to as polypropylene impact copolymers, regardless of the precise nature of their structure, and are said to contain a homopolymer phase (often polypropylene homopolymer) and a rubber phase (the copolymer phase).
A typical process for the production of a polypropylene impact copolymer is conducted in at least two stages in at least two reactors. The homopolymer phase is conventionally produced first in one or more reactors and the product of this first stage together with any unreacted monomer is then passed to a second stage where the copolymer phase is produced. It is in the second stage that a large proportion of the processing difficulties for the overall process are encountered. In slurry, bulk or other solvent/diluent based processes, swelling and partial dissolution of the rubbery second stage product can take place. As a result, the polymer product of the second stage is tacky and adheres to the walls of the reactor and other internal surfaces, e.g., stirring blades. This reaction system fouling is also a problem, in fluidized bed gas phase processes wherein the second stage reactor has a recycle loop in which the unreacted gaseous monomers are passed through a heat exchange unit to remove heat of reaction and then returned to the reactor. Conventional heat exchange units contain a large number of relatively small diameter tubes and these restrictive passages are particularly vulnerable to fouling.
Control of the relative proportions of homopolymer portion and copolymer portion, as well as to some extent the degree of fouling in the second stage reactor, can be effected by the addition of various materials to the reactor. In general, such additives include catalyst deactivators which "kill" or reduce catalyst activity.
In U.S. Pat. No. 4,551,509 Takayuki et al disclose the addition of a polyalkylene glycol to the reaction mixture of a reactor system for producing ethylene homopolymers or copolymers in order to deactivate the catalyst. Levresse et al, U.S. Pat. No. 4,650,841, disclose the use of certain amides, polyalkylene polyols or epoxides for a similar purpose but the additive is introduced into a monomer recycle stream after removal from the stream of a polymer product. Weimer et al, U.S. Pat. No. 3,962,196, employ heterocyclic additives such as N-vinylpyrrolidone to reduce polymer deposits on the interior walls of a polymerization reactor.
In published European Patent Application 225,099 the properties of a polypropylene impact copolymer are said to be improved by catalyst deactivation with a polyalkylene glycol ether in specified quantity relative to the quantity of the titanium component of the polymerization catalyst. The process was a batch, liquid phase process and the glycol ether was introduced in between the first and second stages or added directly to the second stage reactor. As a part of the overall effect of this addition of glycol ether, the activity of the second stage polymerization catalyst is reduced to 30% to 80% of the catalyst activity before addition.
A closely related process is disclosed by Chiba et al, published Japanese Patent Application 8846211, disclosure date Feb. 27, 1988. In this process, which may be gas phase, a polyalkylene glycol ether is added continuously in specified ratio to the titanium to the second stage reactor. The addition may be between the first and second stage reactors or directly to the second stage reactor. In the gas phase modification of the process, preference is stated for adding the glycol ether to the recycle loop downstream from the heat exchanger. In this process as well, the catalytic activity of the second stage polymerization catalyst is reduced to 30% to 80% of the activity before glycol ether addition. The addition to the recycle loop appears to be for convenience only and the addition of the glycol ether to the recycle loop prior to passage through the heat exchange unit is not apparently contemplated.
There are substantial advantages to adding a catalyst deactivator in the recycle loop but prior to reaching the heat exchanger. The polymerization catalyst present in this recycle stream is necessarily small and consists mainly of catalyst fines and particles containing only a partial coating of polymer. As a result, the amount of a catalyst deactivator needed to reduce the activity of the catalyst present in the recycle loop and therefore reduce fouling of the heat exchange tubes is also small. Particularly when the catalyst deactivator is efficient is the activity of the second stage reactor not substantially adversely influenced by any proportion of the catalyst deactivator which is eventually passed to the second stage reactor. It is known from copending U.S. patent application Ser. No. 447,049, filed Dec. 7, 1989, that the addition of small amounts of certain aromatic esters to the recycle loop upstream from the heat exchange unit is beneficial in reducing fouling of the heat exchange tubes without unduly adversely affecting the activity of the second stage polymerization catalyst. The ester ethyl p-ethoxybenzoate has been commercially used for this purpose since late 1988. Nevertheless, it would be of advantage to provide more efficient catalyst deactivators for introduction into the recycle loop between the second stage reactor and the heat exchange unit in order to more effectively reduce the degree of fouling of the heat exchanger tubes.